1. Field of the Invention
The present invention relates to a feeding apparatus and a low noise block down-converter for a waveguide, and more particularly, to a feeding apparatus and a low noise block down-converter, which can simultaneously modify impedance matching at high frequencies and low frequencies and reduce return loss.
2. Description of the Prior Art
Satellite communication has the advantage of wide communication coverage and being free from interference from ground environment, and is widely used for military communication, exploration and business communication services such as satellite navigation, satellite voice broadcast and satellite television broadcast. A conventional satellite communication receiving device consists of a dish reflector and a low noise block down-converter. The low noise block down-converter is disposed at the focus of the dish reflector. After the low noise block down-converter receives radio signals reflected from dish reflector, the low noise block down-converter converts the radio signals down to middle band, and then transmits the radio signals to a back-end radio frequency processing unit for signal processing, thereby providing satellite television programs to users.
Please refer to FIG. 1A that is a schematic diagram illustrating a conventional low noise block down-converter 10 for satellite communication. The low noise block down-converter 10 can be disposed at the focus of a dish reflector to collect radio signals reflected by the dish reflector. As shown in FIG. 1A, the low noise block down-converter 10 consists of a feedhorn 12, a waveguide 14, a septum polarizer 16 and a feeding apparatus 100. The septum polarizer 16 is fixed in the waveguide 14 with a cylindrical shape, and divides the interior of the waveguide 14 in half. FIG. 1B is a schematic diagram illustrating a top view of a front surface of the conventional feeding apparatus 100. The feeding apparatus 100 is utilized to transmit the radio signals received by the feedhorn 12 to a back-end radio frequency processing unit, and consists of a substrate 110, an annular grounded metal sheet 120, a rectangular grounded metal sheet 130, feeding metal sheets 140a, 140b and signal wires 150a, 150b. 
Conventionally, in order to adjust operating frequency range of the low noise block down-converter 10, lengths of the feeding metal sheets 140a, 140b are modified to control impedance of the feeding apparatus 100 so that impedance matching may be achieved with sufficient bandwidth. In practice, however, failures frequently occur—there exists a tradeoff among frequencies. Specifically, please refer to FIG. 1C, which is a schematic diagram illustrating return loss of the feeding apparatus 100 in Ku band (10.7 GHz-12.75 GHz). As shown in FIG. 1C, the return loss of the feeding apparatus 100 is low, merely in a range of 11.00 GHz to 12.00 GHz, while the return loss of the feeding apparatus 100 from 10.7 GHz to 11.00 GHz and from 12.00 GHz to 12.75 GHz is quite high and grows rapidly. Therefore, the feeding apparatus 100 cannot optimize return loss at high frequencies and low frequencies at the same time. Along with the growing needs for satellite television, the number of frequency bands covered by direct broadcast satellites is increasing; as a result, there is an urgent need for improvement in the field.